Starting with the SailFin announcement at last year's JavaOne conference, followed by the release of the Mobicents Sip Servlets implementation mid-January and then, this month, the release of the Cipango application server, I think it's safe to proclaim that this is the year of the open source SIP servlet container.
These free offerings have eliminated a major cost barrier preventing developers from working in the domain of standards-based telecom services. Now the cost of hosting your own IP-based telecom service comes down to the cost of a hosting service or, if you want to bridge to the phone network, then throw in the cost of a VoIP gateway service. At this point, all that's holding back a flood of innovative telecom applications is a ubiquitous VoIP client (perhaps this one?)
And what's the perfect complement to an open source SIP servlet container? Naturally, an open source SIP servlet development framework! ECharts for SIP Servlets makes developing complex converged telecom services much easier than using the raw SIP servlet API. Unfortunately, with open source SIP servlet containers being released so frequently we haven't had a chance to test ECharts for SIP Servlets against them all. Of the three released we've only confirmed that SailFin works but we'll be checking against the other containers in the near future.